FIG. 1 shows a prior gas turbine engine 10. The engine 10 comprises an air intake 12 and a propulsive fan 14 that generates two airflows A and B. A nacelle 30 surrounds the gas turbine engine 10 and defines, in axial flow B, a bypass duct 32.
The gas turbine engine 10 comprises, in axial flow A, an intermediate pressure compressor 16, a high pressure compressor 18, a combustor 20, a high pressure turbine 22, an intermediate pressure turbine 24, a low pressure turbine 26 and an exhaust nozzle 28. The fan 14, compressors 16 and 18, and turbines 22, 24, 26 are connected by respective shafts 34, 36, 38.
An electrical generator 40 is driven by a transmission system, which is in turn driven by one of the shafts 34, 36, 38. The electrical generator 40 provides electrical power to drive at least one of engine accessories (e.g. fuel pumps), and aircraft loads (e.g. environmental control systems (ECS) and aircraft avionics systems). In some cases, several electrical generators are provided. Two or more generators could be driven by the same shaft via a gearbox, or separate generators could each be driven by a separate one of the engine shafts 34, 36, 38.
One prior electrical generator comprises a wound field alternating current electrical generator. The generator comprises a rotor comprising a plurality of electrical windings. The rotor windings are powered by an electrical source, to induce a magnetic field in the rotor. The rotor is surrounded by a stator comprising a plurality of electrical stator windings. As the rotor rotates in use, a rotating magnetic field is produced by the rotor windings, which energises the stator electrical windings to produce an alternating current in the electrical windings of the stator.
The frequency of the electrical power produced by the machine 40 is generally proportional to the rotational speed of the rotor, which will in turn be proportional to the rotational speed of the shaft 34, 36, 38 which drives the generator 40, assuming a constant ratio gearbox is used. Constant speed gearboxes are also available, but these have a significantly higher weight and occupy a larger volume. Consequently, constant ratio gearboxes are generally used in gas turbine engines. The use of a constant ratio gearbox may however be problematic where the electrical load requires a specific frequency range in order to operate. In one example, the aircraft electrical load may be able to accommodate a frequency range of 360 to 800 Hz. This therefore limits the range of rotational speeds of the engine shaft 34, 36, 38 which drives the generator 40. In effect, the minimum rotational speed of the engine 10 must be kept above a predetermined minimum, such that the generator produces alternating current (AC) electrical power having a frequency of at least 360 Hz in this example. As a result of this minimum rotational speed dictated by the electrical requirements of the aircraft, the minimum engine thrust is higher than a thrust that might be achieved if this limitation were not present (i.e. if the minimum engine speed were dependent only on engine stability concerns), leading to increased specific fuel consumption (SFC) in some stages of flight.
One solution is to provide AC electrical power, such as three phase AC power, to the rotor windings, and to control the frequency and phase of the electrical current provided to the individual electromagnetic windings of the rotor, such that a rotating magnetic field is provided from the frame of reference of the rotor. The electrical output frequency FT of the generator is then the product of the rotational frequency FR of the magnetic field produced as a result of rotation of the rotor, and the rotational frequency FC of the rotor magnetic field produced by the AC power provided to the rotor windings:FT=FC+FR 
Consequently, the frequency of the electrical power delivered from the generator output to the aircraft can be altered for a given rotor rotational speed by altering the rotational speed of the magnetic field generated by the rotor windings. The generator 40 can therefore continue to provide power at the required frequency at relatively low rotor rotational speeds, and so relatively low shaft 34, 36, 38, rotational speeds. Such an arrangement is known within the art as a “doubly fed electric machine”, since electrical power is provided to both the stator and rotor windings. Such machines can be used as either generators having a variable electrical output frequency or motors having a variable rotational speed, and are widely used for example as generators in the wind turbine industry.
Alternatively, the output frequency of the generator could be corrected to the required frequency by “power electronics” (i.e. a frequency controller) electrically coupled to an output of the generator. However, power electronics capable of handling the output power of the generator are necessarily heavy, occupy a large volume, and generate a large amount of heat, and so require large cooling systems.
Whichever method is used to correct the generator output frequency, at these relatively low shaft rotational speeds, it has been surprisingly discovered by the inventor that the torque required to drive the generator 40 to produce the required electrical power (in terms of kilowatts electrical kWe) in some cases exceeds the torque capacity of the engine transmission system where the system is retrofitted to an existing system. Alternatively, in a new design, the consequently higher torque will necessarily result in a transmission system having a higher torque capacity, and therefore a higher weight than would otherwise be required. This increased torque is a consequence of the well-known relation that power is equal to torque times rotational speed. Consequently, there is a requirement for an electrical generator for an aircraft that provides sufficient electrical power for the engine and aircraft systems at the required range of frequencies, whilst simultaneously minimising the torque requirements on the drive shaft. Such systems must also comply with aviation standards such as DO-160, published by the Radio Technical Commission for Aeronautics. One such requirement is that the magnitude and frequency of the generated power does not change at a rate above a predetermined rate. Such considerations are known as “power quality”.